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NCV8674
(Rev.:2008)

Very Low Iq Low Dropout Linear Regulator

ON Semiconductor 

NCV8674

Circuit Description

The NCV8674 is a precision trimmed 5.0 V or 12 V fixed

output regulator. Careful management of light load

consumption combined with a low leakage process results

in a typical quiescent current of 30 mA. The device has

current capability of 350 mA, with 600 mV of dropout

voltage at full rated load current. The regulation is provided

by a PNP pass transistor controlled by an error amplifier

with a bandgap reference. The regulator is protected by

both current limit and short circuit protection. Thermal

shutdown occurs above 150°C to protect the IC during

overloads and extreme ambient temperatures.

Regulator

The error amplifier compares the reference voltage to a

sample of the output voltage (Vout) and drives the base of

a PNP series pass transistor by a buffer. The reference is a

bandgap design to give it a temperature−stable output.

Saturation control of the PNP is a function of the load

current and input voltage. Over saturation of the output

power device is prevented, and quiescent current in the

ground pin is minimized. The NCV8674 is equipped with

foldback current protection. This protection is designed to

reduce the current limit during an overcurrent situation.

Regulator Stability Considerations

The input capacitor CIN in Figure 2 is necessary for

compensating input line reactance. Possible oscillations

caused by input inductance and input capacitance can be

damped by using a resistor of approximately 1 W in series

with CIN. The output or compensation capacitor, COUT

helps determine three main characteristics of a linear

regulator: startup delay, load transient response and loop

stability. The capacitor value and type should be based on

cost, availability, size and temperature constraints.

Tantalum, aluminum electrolytic, film, or ceramic

capacitors are all acceptable solutions, however, attention

must be paid to ESR constraints. The aluminum

electrolytic capacitor is the least expensive solution, but, if

the circuit operates at low temperatures (−25°C to −40°C),

both the value and ESR of the capacitor will vary

considerably. The capacitor manufacturer’s data sheet

usually provides this information. The value for the output

capacitor COUT shown in Figure 2 should work for most

applications; however, it is not necessarily the optimized

solution. Stability is guaranteed at values COUT ≥ 22 mF and

ESR ≤ 7.0 W, within the operating temperature range.

Actual limits are shown in a graph in the Typical

Characteristics section.

Calculating Power Dissipation in a Single Output

Linear Regulator

The maximum power dissipation for a single output

regulator (Figure 2) is:

PD(max) + [VIN(max) * VOUT(min)] @

IOUT(max) ) VIN(max) @ Iq (eq. 1)

Where:

VIN(max) is the maximum input voltage,

VOUT(min) is the minimum output voltage,

IOUT(max) is the maximum output current for the

application, and Iq is the quiescent current the regulator

consumes at IOUT(max).

Once the value of PD(Max) is known, the maximum

permissible value of RqJA can be calculated:

RqJA

+

150oC *

PD

TA

(eq. 2)

The value of RqJA can then be compared with those in

thermal resistance versus copper area graph (Figure 26).

Those designs with cooling area corresponding to RqJA’s

less than the calculated value in Equation 2 will keep the

die temperature below 150°C. The current flow and

voltages are shown in the Measurement Circuit Diagram.

75

100

50

10

D2PAK 1 oz

D2PAK 2 oz

D2PAK

25

1

0

0.1

0 100 200 300 400 500 600 700 800 900 0.000001

0.0001

0.01 0.1

Single Pulse

1 10 100 1000

COPPER AREA (mm2)

PULSE TIME (sec)

Figure 26.

Figure 27. NCV8674 @ PCB Cu Area 650 mm2

PCB Cu thk 1 oz

http://onsemi.com

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